1) Before going through the setup steps, be sure to confirm that you have the matching software and firmware installed.
2) After both units have successfully updated, connect the first unit to the first port of the Symphony64 Thunderbridge or PCIe card using a PC-32 cable. Connect one end of the cable to the "main port" on the back of the first Symphony I/O and the other end of the cable to the 1st port (on the left) of the Symphony64 Thunderbridge or PCIe card.
3) Connect the 2nd Symphony I/O to the 2nd port of the Symphony64 Thunderbridge or PCIe card using a PC-32 cable. Connect one end of the cable to the "main port" on the back of the second Symphony I/O and the other end of the cable to the 2nd port (on the right) of the Symphony64 Thunderbridge or PCIe card.
4) Connect the 2 Symphony I/O's to each other using BNC cables with the Loop Sync connectors. Connect "in" from one unit to the "out" of the other and vice versa. Make sure you're using the Loop Sync connectors (on the right side) and NOT the Word Clock connectors (on the left side).
5) Make sure each Symphony I/O is set for "SYM AIM" (Symphony Audio Interface Mode) by pressing in and holding on the right side encoder on the front-panel until it brings you to the AIM menu. If it says "Symphony", it's already in SYM AIM and you can just push the encoder again to exit the menu. If not, scroll through the menu until it says "Symphony" and then push in on the encoder again. The unit will power down and restart itself in SYM AIM. Do this for both units.
6) Open up "Audio MIDI Setup" (Applications Utilities Audio MIDI Setup). Select "Symphony64" in the left-hand device column. Make sure it's selected as your computer's default i/o. The speaker and microphone icons should appear next to it. If they don't, click on the drop-down menu below with the "gear" icon and select both "Use this device for sound input" and "Use this device for sound output".
7) With "Symphony64" selected in Audio MIDI Setup, go to the center part of the Audio MIDI Setup window and set the "Source" drop-down menu to ‘Ports 1-2: 64Chs’. Quit Audio MIDI Setup and restart your computer.